Molecular mechanisms of retinoid action in acute promyelocytic leukemia (Review)

Acute promyelocytic leukemia (APL) is, at present, the first and only example of leukemia which can be induced into remission with a single cyto-differentiating agent. This is due to the fact that APL is exquisitely sensitive to the differentiating action of all-trans retinoic acid (ATRA). Thus, the APL model offers a unique opportunity to study the cyto-differentiating action of ATRA and synthetic retinoids in a clinically relevant setting. This review article summarizes the work relating to the molecular mechanisms underlying the action of retinoic acid and retinoids in APL cells, and focuses on: a) genes which are expressed and regulated by ATRA; b) synthetic retinoids as cyto-differentiating agents; c) rational combinations between retinoids and cytokines or other cyto-differentiating agents; d) cellular paradigms of retinoic acid resistance. It is our aim to give an updated, about nonexhaustive, account of some of the most recent development regarding the pharmacological action of retinoic acid and its derivatives in APL cells.     

The present status in all-trans retinoic acid (ATRA) treatment for acute promyelocytic leukemia patients: further understanding and comprehensive strategy are required in the future.

All-trans retinoic acid (ATRA) has recently been recognized as the first line therapeutic agent in the treatment of patients with acute promyelocytic leukemia (APL). The extraordinary high remission rate achieved by ATRA in comparison with other chemotherapeutic agents suggested that ATRA differentiation induction therapy seemed superior to conventional chemotherapy for APL patients. However, after the great excitement aroused after the initial successes, we have to take stock and examine in detail several problems which have emerged preventing us from improving the clinical outcome in APL. Maintenance in order to prolong remission and prevention of or retreatment for the relapse are the major subjects of concern at present. Efforts should be made either to keep ATRA effective for APL patients or to resensitize the relapsing patients for repeated ATRA therapy. The administration of ATRA should be carefully adapted in accordance with the individual patient's condition. From both conceptual and practical points of view, ATRA differentiation therapy should be combined with chemotherapy, bone marrow transplantation and biomodifier treatment. Thus, a more comprehensive strategy must be planned and developed in the near future. Using molecular biological techniques, the diagnosis of APL can be more precisely made and the course of the disease more closely monitored. The central dogma, still to be revealed, is the relationship between APL pathogenesis, the chromosome translocation present with the relevant molecular alterations and the response to ATRA treatment. Current studies in all these above fields have provided us with a deeper understanding of the pathogenesis of APL and the physiological function and curative action of ATRA.(ABSTRACT TRUNCATED AT 250 WORDS)     

The Present Status in All-trans Retinoic Acid (ATRA) Treatment for Acute Promyelocytic Leukemia Patients: Further Understanding and Comprehensive Strategy are Required in the Future

All-trans retinoic acid (ATRA) has recently been recognized as the first line therapeutic agent in the treatment of patients with acute promyelocytic leukemia (APL). The extraordinary high remission rate achieved by ATRA in comparison with other chemotherapeutic agents suggested that ATRA differentiation induction therapy seemed superior to conventional chemotherapy for APL patients. However, after the great excitement aroused after the initial successes, we have to take stock and examine in detail several problems which have emerged preventing us from improving the clinical outcome in APL.

Maintenance in order to prolong remission and prevention of or retreatment for the relapse are the major subjects of concern at present. Efforts should be made either to keep ATRA effective for APL patients or to resensitize the relapsing patients for repeated ATRA therapy. The administration of ATRA should be carefully adapted in accordance with the individual patient's condition. From both conceptual and practical points of view, ATRA differentiation therapy should be combined with chemotherapy, bone marrow transplantation and biomodifier treatment. Thus, a more comprehensive strategy must be planned and developed in the near future. Using molecular biological techniques, the diagnosis of APL can be more precisely made and the course of the disease more closely monitored. The central dogma, still to be revealed, is the relationship between APL pathogenesis, the chromosome translocation present with the relevant molecular alterations and the response to ATRA treatment. Current studies in all these above fields have provided us with a deeper understanding of the pathogenesis of APL and the physiological function and curative action of ATRA. This will no doubt enhance the development of the ATRA treatment regimen thereby widening the spectrum of leukemias and other neoplasias to be treated with differentiation therapy.     

Treatment of relapsed acute promyelocytic leukemia

By all-trans retinoic acid (ATRA) and chemotherapy over 70% of patients with newly diagnosed acute promyelocytic leukemia (APL) may be cured; 20-30% of patients still relapse and require salvage therapy. For relapsed or refractory APL, a standard treatment has not yet been defined. However, several effective drugs and approaches have been described. Treatment options for relapsed APL include chemotherapy regimens used in the treatment of relapsed acute myeloid leukemia usually combined with ATRA or of other differentiating agents such as liposomal ATRA or synthetic retinoids. Presently, allogeneic peripheral stem cell or bone marrow transplantation is the treatment of choice for younger patients who have a histocompatible donor, as it gives the chance of cure in second or further relapse. For patients without a donor or for those who are not suitable for allogeneic transplantation, autologous stem cell or bone marrow transplantation may offer at least the possibility of a prolongation of remission, if the harvested cells are negative in the RT-PCR of PML/RAR alpha. Arsenic compounds have a high antileukemic effectiveness on APL cells. Arsenic trioxide has recently been approved for relapsed or refractory APL. With this drug, complete hematologic remission rates of 80-92% and long-lasting molecular remissions were achieved in relapsed patients. For patients who do not qualify for these treatment options, monoclonal anti-CD33 antibodies may represent further treatment options.     

Myelodysplastic syndrome after acute promyelocytic leukemia: the European APL group experience.

With improved treatment of acute promyelocytic leukemia (APL) by all trans retinoic acid (ATRA) combined to anthracycline-aracytin chemotherapy (CT), a larger number of those patients may be at risk of late complications. Recently, the Rome group reported five cases of myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML, non-APL) occurring during the course of 77 APL patients (6.5%) in complete remission (CR). From 1991 to 1998, we treated 677 newly diagnosed cases of APL, and 617 of them achieved CR with ATRA combined to CT (n=579) or CT alone (n=38); 246 of them received subsequent maintenance CT with 6 mercaptopurine and methotrexate. With a median follow-up of 51 months, 6 patients (0.97%) developed MDS, 13-74 months after the diagnosis of APL. In all six cases, t(15;17) and PML-RARalpha rearrangement were absent at the time of MDS diagnosis, and karyotype mainly showed complex cytogenetic abnormalities involving chromosomes 5 and/or 7, typical of MDS observed after treatment with alkylating agents, although none of the six patients had received such agents for the treatment of APL. Our findings suggest that MDS can indeed be a long-term complication in APL, although probably at lower incidence than that previously reported.     

Retinoic acid syndrome induced by arsenic trioxide in treating recurrent all-trans retinoic acid resistant acute promyelocytic leukemia

Arsenic Trioxide (As2O3) is an effective agent for treating acute promyelocytic leukemia achieving a complete remission rate of about 60% to 90%. It is similar to all-trans retinoic acid (ATRA) when treating acute promyelocytic leukemia (APL), because both agents have limited side effects compared to conventional chemotherapy, although the treatment period is more prolonged. During treatment, both agents may induce leukocytosis, and in patients taking ATRA, leukocytosis appears to be related to the development of retinoic acid syndrome (RAS). We report here a case of APL treated with ATRA in combination with chemotherapy 3 years earlier. During treatment, an episode of RAS with fever, edema, pericardiac effusion etc. was encountered. Recently, she had a relapse of leukemia, and As2O3 therapy was used. Leukocytosis developed again, and symptoms of fever, skin rash, edema resembling a RAS also developed, which was quickly relieved by steroid administration in a manner resembling response to RAS.     

Combination therapy with arsenic trioxide, all-trans retinoic acid, and gemtuzumab ozogamicin in recurrent acute promyelocytic leukemia

BACKGROUND: From 20% to 30% of patients with acute promyelocytic leukemia (APL) who are treated with all-trans retinoic acid (ATRA) develop recurrent disease. Arsenic trioxide (ATO) is an effective agent for the salvage of patients with recurrent APL, and gemtuzumab ozogamicin (GO) has shown activity in patients with APL. METHODS: The authors investigated the efficacy of a combination of ATO, ATRA, and GO in 8 patients with APL in first recurrence (7 patients with hematologic recurrences and 1 patient with a molecular recurrence). All patients had received previous treatment with ATRA either alone or in combination with other agents. Patients received ATO 0.15 mg/kg intravenously until they achieved a bone marrow complete remission (CR). Once in CR, patients received consolidation with ATO, ATRA, and GO for 10 months. Patients then received maintenance with idarubicin, ATRA, 6-marcaptopurine, and oral methotrexate for 11 months. RESULTS: All 7 patients who had hematologic recurrences achieved CR after a median of 39 days (range, 21-56 days). After a median follow-up of >/=36 months (range, 4-55 months), 6 patients remained alive in CR, and 2 patients died in CR. Six of 8 patients remained in second CR that was longer than their first CR. All 7 evaluable patients achieved molecular remission. There were no grade 3 or 4 extramedullary toxicities. Two patients died, 1 secondary to a complication of metastatic lung adenocarcinoma, and the other of sepsis. CONCLUSIONS: The combination of ATO, ATRA, and GO was effective and may achieve durable remissions in patients with APL in first recurrence. It should be considered as an effective alternative to allogeneic or autologous transplantation.     

Treatment of acute promyelocytic leukemia for older patients

Acute promyelocytic leukemia (APL) represents a remarkable disease in which leukemogenesis is driven by the PML-RARα oncogene and for which targeted treatment with all-trans retinoic acid (ATRA)-based therapy allows substantial chance of cure. APL is seen in a small subset of older patients, with age representing one of the most important prognostic factors for outcome of treatment. Unlike other acute leukemias, the inferior outcomes for APL in older patients relates less to changes in disease biology and more to the increased toxicity of ATRA and the chemotherapy combination regimens used to induce hematologic and molecular responses. Risk-adapted strategies that use less-toxic agents, such as arsenic trioxide, allow treatment of older patients, with greater efficiency and better chances of cure.     

Treatment of acute promyelocytic leukemia with arsenic trioxide: clinical results and open questions

Acute promyelocytic leukemia (APL) is a rare form of acute myeloid leukemia. The specific translocation t(15;17), which results in the fusion gene PML–RARA is the diagnostic and pathomechanistic hallmark of APL. By combination, treatment consisting of the differentiating agent all-trans retinoic acid (ATRA), which targets this molecular lesion, and cytotoxic chemotherapy, cure can be achieved in over 70% of patients. Recently, arsenic trioxide (ATO) has emerged to be the most active single agent in the treatment of APL. Previous studies employing ATO in relapse settings reported average complete remission rates of 85% and a mean overall survival of over 60%. In recent approaches installing ATO in first-line treatment, ATO-induced response rates comparable to previous combination regimen. The results of these newer studies indicate that the backbone of chemotherapy can be dramatically reduced or completely replaced by ATO and ATRA with similar or even better outcome.     

Contemporary Treatment of APL

Acute promyelocytic leukemia (APL) is characterized by coagulopathy, leukopenic presentation and sensitivity to anthracyclines, all-trans retinoic acid (ATRA) and arsenic trioxide (ATO). For the last 25 years, APL has been treated with a combination of ATRA and chemotherapy for induction followed by consolidation and maintenance therapy. This general treatment approach has resulted in cure rates of 80–90 %. ATO, originally approved in relapsed APL, has been incorporated into contemporary upfront treatment regimens with excellent response rates. Recent studies show that most patients with APL can be cured with ATRA and ATO alone, eliminating cytotoxic chemotherapy and resulting in superior outcomes compared to standard treatment. We will herein review historical treatment of APL, treatment considerations in specific patient populations, and therapeutic updates.     

Extramedullary relapse in a patient with acute promyelocytic leukemia: successful treatment with arsenic trioxide, all-trans retinoic acid and gemtuzumab ozogamicin therapies

Acute promyelocytic leukemia (APL) is characterized by the presence of the t(15;17) translocation, resulting in the PML-RAR fusion protein. Standard treatment consists of the combination of all-trans retinoic acid (ATRA) with an anthracycline that results in complete remission (CR) rates in excess of 90%. Recently, several new agents have been shown to have clinical activity in APL. These include a liposomal formulation of ATRA (lipo-ATRA), and gemtuzumab ozogamicin (GO). Herein, we report a patient with APL who relapsed with extramedullary disease 2.5 years after lipo-ATRA therapy and was successfully treated with the sequence of A2O3, ATRA, and GO and we summarize our experience with patients with isolated extramedullary relapse in APL.     

急性早幼粒细胞白血病治疗的进展

 自从应用全反式维A酸（ATRA）和砷剂（ATO）治疗急性早幼粒细胞白血病（APL）以来,APL疗效有了显著提高。初治与复发APL的完全缓解率达80 % ~ 90 %。对APL病理特征的分子遗传学认识的提高为治疗干预提供了理论的基础。近年来为延长无病生存期（DFS）进行了大量的研究,特别在APL新治疗方案和靶向治疗的策略以及新制剂方面的研究,取得良好的结果。     

Retinoid/arsenic combination therapy of promyelocytic leukemia: induction of telomerase-dependent cell death.

Acute promyelocytic leukemia (APL) is efficiently treated with a cell differentiation inducer, all-trans retinoic acid (ATRA). However, a significant percentage of patients still develop resistance to this treatment. Recently, arsenic trioxide (As2O3), alone or in combination with ATRA, has been identified as an alternative therapy in patients with both ATRA-sensitive and ATRA-resistant APL. Previous investigations restricted the mechanism of this synergism to the modulation and/or degradation of PML-RARalpha oncoprotein through distinct pathways. In this study, using several ATRA maturation-resistant APL cell lines, we demonstrate in vitro that the success of ATRA/As2O3 treatment in APL pathology can be explained, at least in part, by a synergistic effect of these two drugs in triggering downregulation of telomerase efficient enough to cause telomere shortening and subsequent cell death. Such long-term low-dose combinatorial therapy strategies, developed also to avoid acute side effects, reinforce the notion that the antitelomerase strategy, based on a combination of active agents, should now be considered and evaluated not only in APL but also in other malignancies.     

All trans retinoic acid in acute promyelocytic leukemia.

All trans retinoic acid (ATRA) is able to induce complete remission (CR) in almost all patients with acute promyelocytic leukemia (APL) through in vivo differentiation of APL blasts. However, it cannot eliminate the leukemic clone and to be effective must be used in combination with anthracycline-based chemotherapy. Experience accumulated over the last 10 years has clearly shown that the combination of ATRA and chemotherapy gives better survival in newly diagnosed APL than chemotherapy alone because of fewer relapses and a higher CR rate experienced by these patients. It is also strongly suggested that maintenance treatment with ATRA, and possibly in combination with low-dose chemotherapy, can further reduce the incidence of relapse. Overall, more than 90% of patients with newly diagnosed APL can achieve CR and about 75% can be cured by the combination of ATRA and chemotherapy.     

Acute promyelocytic leukemia: recent advances in diagnosis and management

Acute promyelocytic leukemia (APL) is a distinct subtype of acute myeloid leukemia (AML) characterized by a specific genetic alteration, affecting the retinoic acid receptor-alpha (RARalpha), and leading to a blockage in the differentiation of the granulocytic cells. The accumulation of the promyelocytic blasts in the bone marrow produces intense peripheral blood cytopenias or, less commonly, hyperleucocytosis, both of which are frequently associated with a life-threatening consumptive coagulopathy. The body of available biological information on APL establishes this leukemia as a unique entity that has to be promptly recognized and clearly distinguished from all other acute leukemias, especially in light of its striking response to treatment with anthracyclines and differentiating agents such as all-trans retinoic acid (ATRA) or arsenic trioxide (ATO). Current state-of-the-art treatments, which include simultaneous administration of ATRA and anthracycline-based chemotherapy for induction and consolidation, as well as ATRA-based maintenance, have dramatically transformed APL into the most curable acute leukemia in adults, with approximately 80% of long-term survivors. Risk-adapted strategies to modulate treatment intensity may be an effective approach to minimizing therapy-related morbidity and mortality while maintaining the potential of cure. Nonetheless, a sizeable proportion of patients will relapse after the ATRA-based upfront therapy. Given the high anti-leukemic efficacy observed with ATO in patients who relapse, this agent is currently regarded as the best treatment option in this setting. In this article, we will review the current treatment strategies in the management of newly diagnosed and relapsed APL. We also highlight other aspects that can be crucial for the outcome of individual patients, including supportive care, recognition and treatment of life-threatening complications, management of ATRA- and ATO-associated adverse events, and the role of minimal residual disease (MRD) monitoring.     

All-trans retinoic acid in the treatment of pediatric acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) is a rare form of acute myeloid leukemia with specific epidemiological, pathogenetic and clinical features. Its frequency varies widely among nations, with a decreased incidence among ‘Nordic’ origin populations. The molecular hallmark of the disease is the presence of a balanced reciprocal translocation resulting in the PML/RAR-α gene fusion, which represents the target of the all-trans retinoic acid (ATRA) therapy. The introduction of ATRA in conjunction with anthracyclines marked a turning point in the treatment of APL, previously associated with a significant morbidity and mortality. Nowadays the standard front-line therapy for pediatric APL includes ATRA in every phase of the treatment, resulting in a complete remission rate of 90–95%. Here we provide an overview of the role of ATRA in the treatment of pediatric APL, summarizing the most relevant clinical results of recent decades and investigating future therapeutic perspectives for children with APL.     

全反式维甲酸联合化疗治疗小儿急性早幼粒细胞白血病

目的观察全反式维甲酸（ATRA）联合化疗治疗小儿急性早幼粒细胞白血病（APL）的疗效。方法22例初治患儿用ATRA诱导治疗;当患儿获完全缓解（CR）后,给予DA方案或IDA方案或HA方案或AA方案巩固治疗3个疗程;以后再用ATRA、化疗交替巩固治疗36个月。结果2例在诱导治疗前死于弥散性血管内凝血（DIC）、颅内出血;22例获CR,CR率100％（20／20）。1、3、5年无病生存（DFS）率分别为100％（20／20）、93．3％（14／15）、84．7％（11／13）。ATRA常见毒副作用依次为皮肤和口唇干燥、头痛、恶心、呕吐、肝功能损害及维甲酸综合征。结论ATRA联合化疗治疗小儿APLCR率高、远期疗效好;在诱导治疗前,DIC、颅内出血仍是APL患儿死亡的主要原因;ATRA毒副作用可耐受。     

Cotylenin A--a plant growth regulator as a differentiation-inducing agent against myeloid leukemia

Acute myeloid leukemia (AML) is characterized by the arrest of differentiation leading to the accumulation of immature cells. This maturation arrest can be reversed by certain agents. Although differentiation therapy for patients with acute promyelocytic leukemia (APL) using all-trans retinoic acid (ATRA) has been established, the clinical response of AML patients other than those with APL to ATRA is limited. We must consider novel therapeutic drugs against other forms of AML for the development of a differentiation therapy for leukemia. Regulators that play an important role in the differentiation and development of plants or invertebrates may also affect the differentiation of human leukemia cells through a common signal transduction system, and might be clinically useful for treating AML. Cotylenin A, a plant growth regulator, is a potent and novel inducer of the monocytic differentiation of human myeloid leukemia cell lines and leukemia cells freshly isolated from AML patients.     

Combined treatment with arsenic trioxide and all-trans-retinoic acid in patients with relapsed acute promyelocytic leukemia.

PURPOSE: Arsenic trioxide (ATO) is capable of inducing a high hematologic response rate in patients with relapsed acute promyelocytic leukemia (APL). Preclinical observations have indicated that all-trans-retinoic acid (ATRA) may strongly enhance the response to ATO. PATIENTS AND METHODS: Between 1998 and 2001, we conducted a randomized study of ATO alone versus ATO plus ATRA in 20 patients with relapsed APL, all previously treated with ATRA-containing chemotherapy. The primary objective was to demonstrate a significant reduction in the time necessary to obtain a complete remission (CR) in the ATO/ATRA group compared with the ATO group. Secondary objectives were safety and molecular response. RESULTS: The CR rate after one ATO with or without ATRA induction cycle was 80%. Clinical and pharmacokinetic observations indicated that the main mechanism of action of ATO in vivo was the induction of APL cell differentiation. Hematologic and molecular response, time necessary to reach CR, and outcome were comparable in both treatment groups. Of 16 CR patients, three patients who reached a molecular remission after one induction cycle had all received chemotherapy for a treatment-induced hyperleukocytosis. Three additional patients who received further additional ATO with or without ATRA cycles converted later to molecular negativity. CONCLUSION: ATRA did not seem to significantly improve the response to ATO in patients relapsing from APL. Other potential combinations, including ATO plus chemotherapy, have to be tested.     

Retinoic acid resistance in acute promyelocytic leukemia.

Primary resistance of PML-RARalpha-positive acute promyelocytic leukemia (APL) to the induction of clinical remission (CR) by all-trans retinoic acid (ATRA) is rare but markedly increases in frequency after > or =2 relapses from chemotherapy-induced CRs. Nevertheless, even in de novo cases, the primary response of ATRA-naive cases is variable by several measures, suggesting involvement of heterogeneous molecular elements. Secondary, acquired ATRA resistance occurs in most patients treated with ATRA alone and in many patients who relapse from combination ATRA chemotherapy regimens despite limited ATRA exposure. Although early studies suggested that an adaptive hypercatabolic response to pharmacological ATRA levels is the principal mechanism of ATRA resistance, recent studies suggest that molecular disturbances in APL cells have a predominant role, particularly if disease relapse occurs a few months after discontinuing ATRA therapy. This review summarizes the systemic and APL cellular elements that have been linked to clinical ATRA resistance with emphasis on identifying areas of deficient information and important topics for further investigation. Overall, the subject review strongly supports the hypothesis that, although APL is an infrequent and nearly cured disease, much can be gained by understanding the complex relationship of ATRA resistance to the progression and relapse of APL, which has important implications for other leukemias and malignancies.